Download Chemistry 140 Name(s):

Chemistry 140
Name(s):
Exercise 2: Accuracy and precision, systematic and random errors
1. The following five mass measurements are made on an electronic balance:
2.331 g, 2.334 g, 2.333 g, 2.337 g, 2.335 g
a. The true mass is 2.390 g. Is this an example of systematic or random error?
b. For the true mass given in part a, write a five measurement sequence (like the
one above) that demonstrates the other type of error.
c. Calculate the average (mean) and the standard deviation of the numbers given
in part a. Don’t forget units or the number of digits that should appear in the
standard deviation.
d. Calculate the average (mean) and the standard deviation of the numbers given
in part b.
e. Suppose the partnership of Alice and Bob measured the sequence of mass
numbers in part a, and the partnership of Carol and Ted measured the sequence
of masses in part b. Given their respective averages and standard deviations, and
assuming you had no knowledge of what they were measuring, would you
conclude that the two groups were measuring the same item? Why or why not?
f. Suggest a way in which Alice and Bob could relatively easily fix their error (or
at least get better results). Why does this fix not apply to Carol and Ted?
2. Classify the following "experimental" errors as causing systematic or random
errors.
a. Reactant powder left in the weighing boat after reactant transfer to the reaction
vessel
b. An electronic balance that is miscalibrated
c. A digital thermometer giving fluctuating readings
d. Calculation error when applying a formula, such as Hess’s Law or the First
Law of Thermodynamics
3. Though words such as "accuracy" and "precision" have precise definitions, in
everyday conversation, they are used as synonyms, which is inaccurate. Classify
the following phrases found in abstracts as either statements concerning
precision or statements concerning accuracy:
a. The heat of reaction was found to be 120.3 ± 2.4 kJ/mol of reactant
b. The measured value was found to be 14% off of the calculated value
c. The wavelength was measured to be 370 ± 10 nm, which was 10% below the
generally accepted result.